Metal halide lamps have been commercially available for about 30 years. Since then, overall lamp performance has been continuously improving. Such improvements include increasing the efficiency and the life of the lamps. Utilization of various materials, including rare earth halides, have yielded substantially higher color rendering index (CRI) at various correlated color temperatures (CCT). More recently, metal halide lamps, with ceramic arc tubes of polycrystalline alumina and compatible special frit materials, have dramatically improved the color consistency of such lamps. The ceramic arc tubes enable metal halide lamps to operate with a much smaller color spread. Also, lamp-to-lamp color variation has been reduced dramatically. Over the last several years, the need to save energy has become more acute due to global warming issues and, as a result, many researchers have been investigating ways of reducing the energy consumption of lighting. Dimming the lamps, of course, generally allows saving energy when full light output is not necessary. In industrial and commercial establishments the dimmed period could be between store closing hours such as 8:00 p.m. or 11:00 p.m. In outdoor applications it could be between 11:00 p.m. and 6:00 a.m. However, when existing metal halide lamps are dimmed, the metal halide vapor pressure in the arc tubes drops dramatically resulting primarily in a mercury discharge in which the CCT of the dimmed metal halide lamp is much higher than the CCT of the lamp when it is operating at rated wattage.
This change of CCT is disturbing and very perceptible to the viewer. R. G. Gibson, "Dimming of Metal Halide Lamps," Journal of IES, Summer 1994, has investigated the issue in lamps which utilize phosphor-coated outer jackets. The phosphor is typically europium-activated yttrium vanadate which responds to the 365 nm emission line of mercury. As a result, when the lamp is dimmed, the mercury UV radiation is converted into more reddish emission from the phosphor coating which results in a relatively small color change. However, this is only for coated lamps and the effects are limited. Clear lamps do not have the advantage of being able to convert mercury radiation into more reddish visible radiation. Coated lamps are more expensive to manufacture and have more limited applications. Therefore, it is highly desirable to have dimmable clear jacket lamps without substantial CCT change under dimming.
In addition to saving energy in high bay and outdoor applications or industrial and commercial installations by turning down the light source, there are also applications in mood control, especially at the lower power levels (e.g., 150 W or lower). This could be in applications where metal halide lamps are used primarily indoors, such as restaurants, residential and semi-commercial installations or entertainment establishments, where color has to be maintained under continuous dimming.
A further application for dimmable metal halide lamps is where one wants to maintain constant light output throughout the life of the lamps. As is well known, under normal circumstances, where the lamp is operated at rated power over its life, there will be some light output depreciation which can be as much as 40-50% depending on the maintenance, chemistry and power level of the lamp. In some applications this is quite objectionable and the environment is not well served by that kind of light output depreciation. Therefore, compensation could be produced by either overdriving the lamp (which shortens the life considerably) or starting from a 20-30% reduced power state, so as luminosity decreases, lamp power increases automatically to produce a constant light output through the life of the lamp. This can be done with electronic power controls and sensors.
When commercially available metal halide lamps are dimmed, typically the color temperature increases from 500.degree. K to about 1500.degree. K, depending on the lamp chemistry. Furthermore, one of the dramatic changes that takes place (especially for rare earth chemistries) is the hue of the light source changes from white to somewhat greenish. Such lamps contain (in addition to rare earth halides) thallium iodide and sodium iodide to improve the efficacy. The vapor pressures of the rare earth halides are substantially different than thallium iodide at different temperatures. Thallium iodide typically is unsaturated and has a higher vapor pressure than the vapor pressures of the rare earth halides. When the lamps are dimmed 30 or 50% in power, the cold spot temperature drops. The less volatile rare earth halide vapor pressure drops dramatically, whereas the thallium iodide remains in the gas phase and gives a greenish hue to the lamps. When thallium iodide is removed so only rare earth halides remain, the efficiency is not as high as it could be, or has been shown to be possible, under full rating of the lamps.